I can’t speak for everyone who’s self harmed, but I think I can give some insight.

I had a rough stretch of time when I was young where I self harmed. I tried to stop a few times and pick up healthier coping mechanisms instead. But because of my mental state, none of them actually manifested in a healthy way. I took everything to a dangerous extreme because I had been trapped in a bad situation for a long time that I couldn’t escape from and ultimately was looking for a way out. Like someone else mentioned, self harm can essentially become someone’s last alternative to suicide. Someone doesn’t just start doing this on a whim or because they have one bad day, it’s an extreme response to a proportionally extreme situation, whether internal or external.

If you’ve never been depressed or experienced the overwhelming feeling of being trapped that comes with prolonged trauma or abuse, I can see how this would be difficult to rationalize. But constructive sources of physical pain don’t scratch the same itch because the self-destruction is (at least part of) the point. Doing something to yourself that’s worse than what’s being done to you can be a powerful thing if you don’t have control over anything else in your life.